A programmable read only memory (PROM) of the prior art is most conveniently thought of as being a non-volatile memory, such as a diode matrix, with a known number of parallel input ports and a known number of output ports. When a user acquires the PROM, any combination of logic signals applied to the parallel input ports causes all of the output ports to provide logic signals representative of `1` (logic `1`).
The signals applied to the parallel input ports are usually referred to as an address. The address is said to be in parallel because signals comprising the address are applied simultaneously to the parallel input ports.
The non-volatile memory has signal paths that are connected together by fusible links. The user applies a voltage via an electronic programming apparatus to a fusible link to blow the link and thereby cause a disconnection between two signal paths of the non-volatile memory. Because of the disconnection, selected addresses cause a logic signal representative of `0` (logic `0`) to be provided at one or more of the output ports. By selectively blowing the links, the output ports provide a predetermined group of logic signals, referred to as a data word, in response to an address.
In automotive applications, for example, the address may be representative of phenomena in any one of a plurality of spacially separated locations, such as temperature of a bearing and air in a fuel mixture in a carburetor. It is inconvenient to provide signal lines from the parallel input ports to each of the locations.
It is preferable if, for example, one signal line is connected from each of the locations via a multiplexer to a serial input port. That is to say, each of the logic signals of the address is serially applied, one at a time, at a serial port.
Prior art PROM's do not have the capability of handling data transfer and addressing of the non-volatile memory from both a serial and a parallel port.
Such capability, when required, was expensive due to the large number of individual IC's, less reliable due to the large number of inter-connections and cumbersome due to the large space required.